A passenger with measles rode the BART during rush hour for three straight days

SAN FRANCISCO (Reuters) - Commuters on San Francisco's Bay Area Rapid Transit system may have been exposed to measles last week after an East Bay resident with the disease rode a BART train to and from work in the city for three days while infectious, public health officials said on Wednesday.

Health authorities said the overall chance of contracting measles by being exposed on a BART train was low, but anyone who has not been previously vaccinated or has not had the disease before is at relatively high risk of getting it if exposed. More than 100 people in California have been diagnosed with measles since December, fewer than 10 of them in the Bay Area.

In a statement released on Wednesday, Contra Costa Health Services specified that the measles-infected passenger "traveled between the Lafayette Station and Montgomery station in San Francisco during the morning and evening commutes from 6 a.m. to 8 a.m. and 7 p.m. to 9 p.m. on Wednesday, Thursday and Friday, February 4-6." But measles is extremely contagious, so it's not just unvaccinated people who sat right next to the passenger who are at risk.

"Because the measles virus can stay in the air for up to two hours and BART cars circulate throughout the Bay Area, anyone who used the transit system during that time could have potentially [been] exposed to the virus," the statement said.

The person with measles also spent time at E & O Kitchen and Bar during the evening of Wednesday, February 4, and health officials are investigating so they can track the rest of his or her contacts and movements before diagnosis.

"Measles is circulating in the Bay Area and we don't know yet where this person was exposed," Erika Jenssen of Contra Costa Public Health said in the statement.

People who are up-to-date on their vaccinations do not need to worry, as their risk of contracting measles is extremely low even when directly exposed. But people who are unvaccinated, either because they have refused vaccination or because they are too young or otherwise ineligible for vaccination, will almost always contract measles when exposed.

"While we are concerned about the current outbreak in California and its potential to spread, we cannot emphasize enough that the solution is simple and available," Dr. Tomas Aragon, San Francisco Health Officer, in the statement. "Be vaccinated."

(Reporting by Noel Randewich in San Francisco; Writing By Steve Gorman; Editing by Cynthia Johnston)